Biology subjects

Bracewell, R. R.

Publications and source records attributed to Bracewell, R. R..

5 recordsLinked to original sources

bioRxiv2026

Genome reshuffling as a route to specialization? Chromosome-level insights from the bark beetle symbiont genus Geosmithia.

Evolution of genome architecture is increasingly recognized as a major driver of fungal adaptation. However, because most studies have focused on plant and human pathogens, the genomic mechanisms underlying adaptation beyond pathogenic lifestyles remain poorly understood. Here, we establish the bark beetle-associated fungal genus Geosmithia as a model for studying genome evolution during ecological transitions. Geosmithia species occupy diverse ecological niches ranging from generalists, facultative symbionts of multiple beetle vectors and host tree species to more specialized conifer specialists and obligate nutritional symbionts of ambrosia beetles. Using chromosome-level genome assemblies of eleven species representing independently evolved ecological strategies, we investigated how genome architecture, repetitive DNA, and gene family evolution contribute to this ecological diversification. Comparative genomics revealed pervasive intra- and interchromosomal rearrangements, even between recently diverged species, demonstrating that extensive chromosome restructuring can accumulate over short evolutionary timescales. Breakpoints frequently occurred in gene-rich rather than repeat-rich regions, contrasting with the transposable element-associated patterns commonly described in fungi. Hi-C analyses revealed a typical Rabl chromosome organization but highly unusual centromeres that are repeat-poor, gene-rich, transcriptionally active, and conserved as syntenic blocks across species. Further, centromere identity may be determined by a highly divergent CenH3 variant. Despite similar genome sizes, specialists had increased repetitive DNA content and widespread gene family contractions, consistent with metabolic streamlining during ecological specialization. Our results demonstrate that extensive structural genome evolution can occur in gene-dense fungal genomes and establish Geosmithia as a powerful model for studying the interplay between genome evolution and ecological diversification.

genomics↗

bioRxiv2026

Hidden in the genomic bycatch: insights into genome reorganization and population structure of the parasitic nematode Contortylenchus reversus

Insect-parasitic nematodes are widespread and often significantly reduce host fitness, yet we know surprisingly little about most species. Contortylenchus reversus is a hemocoel-inhabiting parasitic nematode that infects Dendroctonus bark beetles, notably impacting host mobility and fecundity. We first detail a chromosome-scale genome assembly of C. reversus, recovered serendipitously from a sequencing project targeting a host (Dendroctonus ponderosae). We assembled the 79.4 Mb genome into nine linkage groups and, through transcriptome-aided annotation, identified 11,244 protein-coding genes. Synteny comparisons with the only relatives for which there are complete assemblies reveal extensive chromosomal rearrangements and extreme loss of gene collinearity suggesting these insect-parasitic nematodes may have exceptionally malleable genomes. Using this genome assembly and repurposed reduced-representation genomic data from 707 D. ponderosae individuals, we investigated infection frequencies and population structure, identifying infection rates ranging from 0% to 50% across 18 geographically widespread collection sites. Population structure of C. reversus appears broadly concordant with the structure of the host beetle, suggesting a shared evolutionary history, while genetic variation (nucleotide diversity) in the parasitic nematode is highly reduced in comparison to its host. These results offer insights into its population genetics, host associations, and the evolutionary dynamics of a nematode-beetle interaction and highlight how genomic bycatch can reveal previously hidden details about an important species in a complex community.

genomics↗

bioRxiv2026

Species-rich and genomically diverse: comparative genomics reveals how fusions, fissions, and sex chromosomes have shaped beetle evolution

Chromosome evolution in animals reflects a balance between long-term conservation of ancestral linkage groups and lineage-specific chromosomal rearrangements that reshape genome structure. Beetles (Coleoptera), the most species-rich animal order, exhibit extensive diversity in karyotype, yet the extent to which their chromosomes retain deep ancestral structure remains unclear. Here, we analyzed 190 chromosome-level genome assemblies spanning 39 families and 16 superfamilies to characterize genome diversity, evaluate the conservation of ancestral linkage groups (Stevens elements), identify neo-sex chromosomes, and explore the role of repetitive elements in driving karyotypic change. Our results reveal that beetle genomes are highly diverse, varying substantially in genome size, chromosome number, GC content, and transposable element (TE) composition. Despite this diversity, Stevens elements appear conserved across much of the radiation, with several superfamilies maintaining strong chromosome synteny over more than 200 million years of evolution. In contrast, some clades, and specifically the leaf beetles (Chrysomelidae), have undergone extensive genomic changes including numerous chromosomal fusions and fissions and changes in genome size. Using synteny based approaches across beetles, we identified 37 species (approximately 19.5%) having patterns consistent with neo-sex chromosomes, a substantially higher frequency than previous estimates. These putative X-autosome fusions vary in complexity and age, often clustered within lineages that are prone to chromosomal instability. The ancestral X chromosome appears conserved for over 300 million years, with stable gene content and reduced TEs relative to autosomes. These findings help establish beetles as a promising system to uncover the evolutionary forces that maintain and disrupt ancestral linkage groups and drive the formation of neo-sex chromosomes.

genomics↗

bioRxiv2026

The tortured past of young polymorphic sex chromosomes revealed through multiple de novo genome assemblies of the mountain pine beetle

Neo-sex chromosomes provide a powerful system for studying the early stages of sex chromosome evolution and the genomic mechanisms that may contribute to reproductive isolation. Using PacBio long-read HiFi sequencing, Hi-C scaffolding, and sex-specific transcriptomic data, we generated six chromosome-level assemblies (male and female from three populations) of the mountain pine beetle (Dendroctonus ponderosae), a species known to harbor three partially reproductively isolated neo-Y haplogroups. These assemblies reveal that the large neo-X and neo-Y chromosomes formed through sequential fusions of the ancestral X with three autosomes, with recombination cessation occurring at [~]8.6, [~]6.3, and [~]4.3 MYA for each event. Comparative analyses show that while neo-X chromosomes remain largely collinear across populations, neo-Ys exhibit dramatic structural divergence, with 900-1,200 inverted segments per haplogroup and only [~]65% of sequence able to be aligned to the neo-X. Repeat analyses demonstrate moderate TE accumulation on the neo-Y, particularly LTR elements, and gene mapping analyses reveal extensive degeneration: [~]62% of neo-Y genes exhibit gene loss, fragmentation, or disruptive mutations. All populations retain a single pseudoautosomal region (PAR), though PAR size and gene content vary due to neo-Y specific rearrangements. Across neo-Ys, 27 genes are uniquely missing in the Western haplogroup, including previously identified candidates implicated in hybrid male sterility. Broader comparisons among neo-Ys show widespread structural variation, population specific patterns of degeneration, and limited gene family expansions. Together, these results provide the first full characterization of neo-sex chromosome evolution in D. ponderosae, revealing rapid, lineage specific neo-Y degeneration and highlighting the potential for sex chromosome divergence to contribute to emerging reproductive incompatibilities within a single species.

genomics↗

bioRxiv2023

A chromosome scale genome assembly and evaluation of mtDNA variation in the willow leaf beetle Chrysomela aeneicollis

The leaf beetle Chrysomela aeneicollis has a broad geographic range across Western North America, but is restricted to cool habitats at high elevations along the west coast. Central California populations occur only at high altitudes (2900-3450 m) where they are limited by reduced oxygen supply and recent drought conditions that are associated with climate change. Here we report a chromosome-scale genome assembly alongside a complete mitochondrial genome, and characterize differences among mitochondrial genomes along a latitudinal gradient over which beetles show substantial population structure and adaptation to fluctuating temperatures. Our scaffolded genome assembly consists of 21 linkage groups; one of which we identified as the X chromosome based on female/male whole genome sequencing coverage and orthology with Tribolium castaneum. We identified repetitive sequences in the genome and found them to be broadly distributed across all linkage groups. Using a reference transcriptome, we annotated a total of 12,586 protein coding genes. We also describe differences in putative secondary structures of mitochondrial RNA molecules, which may generate functional differences important in adaptation to harsh abiotic conditions. We document substitutions at mitochondrial tRNA molecules and substitutions and insertions in the 16S rRNA region that could affect intermolecular interactions with products from the nuclear genome. This first chromosome-level reference genome will enable genomic research in this important model organism for understanding the biological impacts of climate change on montane insects.

genomics↗